3H-Codeine binding in the guinea pig lower brain stem.

Saturable binding of (-)-3H codeine was found in the guinea pig medulla (KD = 5.6 x 10(-7) M, Bmax = 1.4 pmol/mg protein), whereas little stereospecific binding was detected (KD = 4.4 x 10(-5) M). The saturable binding of (-)-3H codeine was slightly enhanced by Na+ and by Mg++ but not by Li++ and Ca++. The enhancement appears to be due to an increase in the number of receptor sites. (-)-3H-Codeine binding was displaced by (-)- and (+)-codeine, morphine, (-)- and (+)-methadone but not by barbiturates. Naloxone, at a high concentration (1 x 10(-5) M), inhibits the binding by only 40%. This agrees with our previously published data which shows that the optical isomers of codeine had significant antitussive effects in the cat, these effects not being antagonized by naloxone. A class of opiate antitussive receptors, which are less naloxone-sensitive and less stereoselective than the mu receptors, is implicated.